1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O2, NOx, or CO, and more particularly to an improved structure of such a gas sensor which provides for ease of installation of a protective cover on a cover mount, firm engagement of the protective cover with the cover mount, and ease of machining of the cover mount.
2. Background Art
Typical air-fuel ratio feedback control systems for automotive internal combustion engines usually employ the concentration of oxygen contained in exhaust gasses measured by a gas sensor installed in an exhaust system. A gas sensor of such a type usually includes a hollow cylindrical housing, a sensor element disposed within the housing, and a protective head cover projecting from an end of the housing to cover a head of the sensing element. The protective head cover is made of a metallic member for protecting the fragile sensor element against undesirable impacts and usually has a double walled structure consisting of an outer and an inner cover wall.
The installation of the outer and inner cover walls on the housing is accomplished by placing ends of the outer and inner cover walls so as to overlap each other on a side surface of an end portion of the housing and laser-welding the periphery of the ends of the outer and inner cover walls to the housing.
For example, Japanese Patent First Publication No. 9-304332 teaches such installation which will be discussed below with reference to FIG. 11.
A gas sensor 9 includes a housing 10, a cup-shaped sensing element 29, an outer cover wall 91, and an inner cover wall 92 which have dimples 911 and 921 formed in bottoms 910 and 920 thereof. The inner cover wall 92 is fitted within the outer cover wall 91 in engagement of the dimple 921 with the dimple 911.
The housing 10 has a small-diameter portion 15 formed on an end portion 106 thereof. The outer and inner cover walls 91 and 92 have open end portions 915 and 925 which are so formed as to be lapped one over the other when the inner cover wall 92 is fitted within the outer cover wall 91 in engagement of the dimple 921 with the dimple 911. The open end portions 915 and 925 are welded as indicated at 14, to the periphery of a side surface 151 of the small-diameter portion 15 of the housing 10.
The dimples 911 and 921 which work to secure the lap of the open end portions 915 and 925 of the outer and inner cover walls 91 and 92 are formed on the bottoms 910 and 920 remotest from the open end portions 915 and 925. Therefore, if the distance between the open end portion 915 and the bottom 910 of the outer cover wall 91 or between the open end portion 925 and the bottom 920 of the inner cover wall 92 is changed from a set value due to some production errors, it will result in instability of the lap of the open end portions 915 and 925 and the side surface 151 of the housing 10. If the outer cover wall 91 is short of length, it may not be welded to the housing 10 firmly, which causes, in the worst case, the outer cover wall 91 to be separated from the housing 10 during use of the gas sensor 9.